Why a heat pump gives more heat than the power it uses
A heat pump moves heat from the outdoor air into your room instead of making it. BRANZ says that's why it gives the same heating as a standard electric or gas heater for much less energy.
The measure is the coefficient of performance (COP): heat out for each unit of electricity in. Consumer NZ says a good heat pump has a COP of 3 or more, so 1 kW of electricity becomes 3 kW of heat, where a plug-in heater gets slightly less than 1 kW of heat from 1 kW.
Homes aren't laboratories, though. BRANZ says a unit rated at a COP of 2.0 to 4.5 may only manage around 1.5 to 3.0 overall in real use, and air-to-air heat pumps lose efficiency as it gets colder outside.
Two ways to estimate your own
EECA's way: the label's yearly figure
Find the Cold zone on the Zoned Energy Rating Label, which is the zone for all of New Zealand. Add the heating kWh per year to the cooling kWh per year, then multiply by your rate per kWh. EECA says those label figures are an indication only, since every household uses its unit differently.
The quick way: power in, times hours, times your rate
Find the input power in kW on the spec sheet, or the Power Input kW line on an older Energy Rating Label. Multiply it by the hours you expect to run the heat pump, then by your rate per kWh. As arithmetic only: 2 kW for 3 hours is 6 kWh.
Know what the quick way overstates
Input power is what the unit draws flat out. Mitsubishi Electric NZ says a heat pump ramps down once the room reaches the set temperature, so most hours use less. EECA says the new label's yearly figures are more useful to buyers than the old full-load power draw.
Use your own rate
Your power bill shows the rate per kWh. If your plan has different rates at different times of day, use the one for the hours you'll actually run the heat pump.
Or let the tool do the sums
The running cost tool does the quick way for you: the power input, the hours a day you run it, and the rate you type in from your bill. It doesn't assume a rate.
What pushes the number up
Cold weather. Below 5.5 degrees outside, EECA says the unit has to divert some heating capacity to defrosting its coils, and Consumer NZ notes that de-icing takes extra energy. Frosty towns use more power for the same warmth.
The wrong size. BRANZ says an undersized heat pump runs continuously trying to reach the set point, while an oversized one cycles on and off, which reduces efficiency.
A high setting. Mitsubishi Electric NZ recommends 18 to 22 degrees on heat, and says turning it up to 28 won't warm the room any faster, it just makes the heat pump work harder. Rinnai NZ suggests the same 18 to 22 degree range in winter. Check your own manual too.
Restricted airflow. Consumer NZ says the filters need regular cleaning to keep the unit at maximum efficiency, and Mitsubishi Electric NZ says anything that compromises airflow around the outdoor unit compromises its performance.
Leaky or overlong ducts. On a ducted system, BRANZ says ducts that are overlong, have many bends or run through uninsulated space can lose more than 30% of the heating energy.
Heating an empty house. Consumer NZ's product test team leader says you should only have your heat pump running when you're home. For most New Zealand homes, he says, leaving it on all day wastes energy on heat that leaks out.
Settings and habits that bring it down
- Use Heat mode in winter rather than Auto, as Mitsubishi Electric NZ suggests. Dry mode doesn't heat at all.
- Set the timer to start around when you lose the sun, or about 30 minutes before you get home (Mitsubishi Electric NZ).
- If it runs overnight, try a lower setting. Mitsubishi Electric NZ gives 16 to 18 degrees as an example.
- Close the curtains while it heats, and shut the doors to rooms you aren't using.
- Clean the filters the way your manual describes, and keep plants and clutter clear of the outdoor unit.
- For damp air, Consumer NZ says a purpose-built dehumidifier works better than running the heat pump on Dry.
How heat pumps compare with other heaters on efficiency
| Heat pump | Electric heater | Wood burner or flued gas heater | |
|---|---|---|---|
| BRANZ's efficiency group | Top group: COP usually above 1 | COP about 1 | COP below 1 |
| Heat from 1 kW of energy | About 3 kW at a COP of 3 (Consumer NZ) | Slightly under 1 kW for a plug-in heater (Consumer NZ) | Less than 1 kW of useful heat |
| Worth knowing | Efficiency drops as the outdoor air gets colder (BRANZ) | Turns 100% of its electricity into heat (BRANZ) | Every new wood burner install needs a building consent (BRANZ) |
Don't switch to an unflued gas heater to save power
Portable LPG cabinet heaters and other unflued gas heaters burn gas inside the room. BRANZ doesn't recommend any kind of unflued gas heater, natural gas or LPG, for health and safety reasons, because combustion byproducts like carbon monoxide and nitrogen dioxide need venting outside. They also never count towards the healthy homes heating standard in a rental.
Will a heat pump lower my power bill?
Not necessarily, and BRANZ is upfront about it: installing a heat pump isn't likely to reduce heating costs. In a BRANZ study of 160 households with heat pumps, more said their energy costs had gone up than down, and only 15% described running costs as excellent.
The same study found people kept their homes warmer than before. In other words, plenty of households spend the efficiency on comfort. That's not a bad trade, but it's worth knowing before you budget around savings.
If your house leaks heat, deal with that first: BRANZ says heat pumps don't replace good insulation. Our page on heat pumps compared with other heating covers the alternatives.
What you can do yourself
You can
- Read the Cold zone kWh figure on the label.
- Find your rate per kWh on your power bill.
- Set the temperature and the timer the way your manual suggests.
- Clean the filters and keep the outdoor unit clear.
- Close curtains and doors while it heats.
Leave to a licensed installer
- Checking a unit that runs flat out and never seems to catch up.
- Finding and fixing a refrigerant leak.
- Moving an outdoor unit that's boxed in or on a cold, shaded wall.
- Fixing long, kinked or leaky ducts on a ducted system.
What we'd do
Where we'd start
We'd estimate it EECA's way first, with the Cold zone kWh figure and our own rate, because it's built around a year of use rather than one flat-out moment. Then we'd set the heat pump between 18 and 22 degrees, use the timer instead of leaving it on all day, and clean the filters. If it still seems to run hard all winter without keeping up, we'd have a heat pump servicing specialist check the size and the outdoor unit.